Voters in five federal electorates – Longman, Braddon, Fremantle, Perth, and Mayo – will head to the ballot box for by-elections on July 28 in the wake of the dual citizenship fiasco.

The date, chosen by the government, has infuriated Labor because the date is right in the middle of the party’s triennial national conference.

Labor is now looking at moving the Adelaide conference to focus on the long, 90-day election campaigns.

Deputy leader Tanya Plibersek accused Prime Minister Malcolm Turnbull of “a sneaky tactic” when speaker Tony Smith, announced the date after Question Time on Thursday, blaming the delay on school holidays and new rules Australian Electoral Commission (AEC) to clarify the citizenship status of candidates.

“This is a disgraceful delay,” Plibersek said.

“It would appear this has been deliberately designed to disadvantage the Labor Party, given our national conference is scheduled for that weekend.”

Four of the five seats, across four states, are held by the ALP seats. The party sits on narrow leads in two: Tasmania’s Braddon (Justine Keay), and Queensland’s Longman (Susan Lamb) are marginal at 2.2%, and 0.8%, respectively. One Nation preferences helped Lamb win in 2016, a benefit that won’t come from Pauline Hanson’s party again.

because they were dual citizens. Three of those seats are held by Labor. The fourth, Mayo in South Australia by independent Rebekha Sharkie of Centre Alliance (the former NXT).

All four are being recontested by the previous MPs.

The Perth byelection was caused by at the start of May.

Electoral commissioner Tom Rogers appeared before a Senate estimates committee after the date was revealed and Labor Senate leader Penny Wong said the date looked “partisan”.

Rogers said he “provided independent advice to the Speaker” and the long lead time was “to give every candidate time to put together information they need to comply with the new requirements”.

“We got the final copy of the regulations this morning, and even putting aside the time the AEC requires, candidates require some time to prepare for this process,” he said.

A letter dated May 23 from Rogers to the Speaker said July 28 was “optimal” because it “provides sufficient time for the AEC to implement the changes” and also “ensures that voters are not disenfranchised” because of school holidays, arguing it could also advantage the major parties over the independents.

Four weeks earlier on June 30 was a possibility, and the AEC said it was ready to hold the by-elections on that date, but in the end July 28 was of the government’s choosing.

The Coalition does not plan to contest the two by-elections in WA, but the long campaign will no doubt challenge the coffers of Sharkie in Mayo, a seat previously held by the government.

ALP National President and Port Adelaide MP Mark Butler said the by-election date “stinks of interference by Turnbull” and “appears to have been deliberately designed to disadvantage the Labor Party, given our National Conference is scheduled for that weekend”.

“This delay will leave about 500,000 Australians unrepresented for nearly three months,” he said.

“The National Executive will now consider the implications of today’s decision for our Conference, and confirm arrangements in the near future.”